Referring to FIG. 1, a conventional detecting device for detecting presence or level of a substance in a container 19 is shown to include a housing 11 accommodating the container 19, a transmitting module 12 disposed in the housing 11, and a receiving module 13 disposed in the housing 11. The transmitting module 12 is operable to transmit a photo signal to pass through the container 19, and the receiving module 13 is configured to receive the photo signal transmitted from the transmitting module 12 for determining whether the substance is present in the container 19. However, the transmitting module 12 and the receiving module 13 of such conventional detecting device need to be correspondingly disposed on two opposite sides of the container 19, thereby requiring a relatively large space for installation of the same. Moreover, a distance between the transmitting module 12 and the receiving module 13 is relatively long, which may result in false detection due to insufficient strength of the photo signal received by the receiving module 13.
Referring to FIG. 2, another conventional detecting device is disclosed, which includes a laser-emitting module 14 for transmitting a laser signal to pass through a container 17, and a laser-receiving module 15 for receiving the laser signal and for determining whether the substance is present in the container 17. Although utilization of the laser-emitting module 14 and the laser-receiving module 15 provides a greater maximum detecting distance between the laser-emitting module 14 and the laser-receiving module 15 compared with that of the above-mentioned conventional detecting device in FIG. 1, manufacturing costs and energy consumption of the laser-emitting module 14 and the laser-receiving module 15 are relatively high.